Recently, electronic devices such as computers, mobile phones, personal digital assistants (PDAs), or the like having an input device for allowing a finger to make contact therewith to operate the electronic devices have been mainstreamed. As such an input device, a resistive film type touch panel (touch sensor) or the like is known.
FIG. 1 schematically illustrates a resistive film type touch panel. A resistive film type touch panel (hereinafter, simply referred to as a “touch panel”) 100 has a first resistive film 102, a second resistive film 104, a pair of X electrodes 106, and a pair of Y electrodes 108. For the sake of explanation, the X-axis and the Y-axis are taken as the directions of illustration. The first resistive film 102 and the second resistive film 104 are disposed to face each other with a gap therebetween. The pair of X electrodes 106 is formed along two opposite sides of the first resistive film 102 extending in the Y direction. Further, the pair of Y electrodes 108 is formed along two opposite sides of the second resistive film 104 extending in the X direction. Wirings (terminals) XP and XN are drawn from the pair of X electrodes 106, and wirings YP and YN are drawn from the pair of Y electrodes 108.
When a user makes contact at an arbitrary point (contact point) PT, the first resistive film 102 and the second resistive film 104 contact each other at that point PT. The contact resistance is indicated by RC. When detecting an X coordinate, a constant voltage (driving voltage) VDRV is applied between the pair of X electrodes 106 so that an electric potential Vx is generated at the contact point PT. The electric potential Vx is obtained by dividing the constant voltage VDRV by resistors Rx1 and Rx2.Vx=VDRV×Rx1/(Rx1+Rx2)
The resistors Rx1 and Rx2 have values corresponding to the X coordinates, respectively. When the wirings YP and YN have high impedance, the electric potential Vx of the contact point is observed as an electric potential of the wirings YP and YN through the second resistive film 104. When detecting a Y coordinate, a constant voltage VDRV is similarly applied between the pair of Y electrodes 108 so that an electric potential Vy of the first resistive film 102 is measured.Vy=VDRV×Ry1/(Ry1+Ry2)
A touch panel that is compatible with multi-touch has been required in recent years. As a distance between two points of multiple touches is longer, the combined impedance of the first resistive film 102 and the second resistive film 104 is decreased. Thus, it is possible to detect coordinates of two points by detecting the combined impedance.
Here, a resistance value of a resistive film and contact resistance is changed depending on the temperature and aging. Further, the amount of change varies depending on a material or structure. Thus, in the related art touch panel, there was a problem that, when a period of time has lapsed, errors of coordinates become large, especially at the time of multi-touch. In this context, once the panel is deteriorated, it was necessary to recall the product, calibrate it, and release it again.